The present invention relates generally to sewage treatment, and, more particularly, to a wastewater treatment system in which both aerobic and anoxic treatment functions are carried out in a single vessel.
It is well known in the relevant art that the most effective treatment of sewage and other organic-laden wastewater employs digestion by both aerobic and anaerobic microbes. During the aerobic stage (during which the liquor is usually aerated by artificial means), a first group of bacteria metabolizes the waste and carries out nitrification functions. Denitrification is then performed by a second group of bacteria during the anoxic (sometimes referred to as the anaerobic) phase. After the waste has been treated through one or more complete nitrogen cycles the liquid is separated from the microbes by decanting the liquid and transferring it out of the system.
Most prior art systems of this type have employed at least two separate treatment tanks, one for the aerobic phase and the other for the anoxic. However, the dual-tank approach results in duplication of components and the need for extra monitoring and transfer equipment, and so adds greatly to the capital costs, space requirements, operating expenses, and maintenance requirements of these systems.
Because of these problems, there have been some previous attempts to develop systems in which both the aerobic and anoxic treatment phases are carried out in a single tank or vessel. While some of these systems have had a degree of success when used with single-family dwellings and other small installations, in general they have not proven suitable for larger installations, such as industrial or municipal wastewater treatment facilities. Where there are comparatively large influent flows, these tend to stir up the sludge from the bottom of the tank, causing the waste solids and influent bacteria to become suspended in the water column and rendering it impossible to safely decant the liquid.
One type of system which has attempted to deal with this problem is disclosed in U.S. Pat. Nos. 5,186,821, 5,234,580, and 5,384,049, all to D. Thomas Murphy. Each of these systems includes a collector or chamber having baffles which slow the influent flow, so that the solids settle out and create a xe2x80x9cprestratification zonexe2x80x9d above the sludge layer (see area 27 in the Murphy patents); the fluids pass through the prestratification zone and are xe2x80x9cfilteredxe2x80x9d before entering the tank. The system also includes a programmable timer for cyclical aeration, settling and decanting of the tank.
As noted above, however, the systems shown in the Murphy patents are believed to be suitable only for fairly small installations, such as for a single family dwelling (similar to a septic tank), in which the decant rate is typically on the order of 10 GPM. For example, the xe2x80x9cprestratification zonexe2x80x9d is limited by the cross-sectional area available in the lower end of the collector, and cannot hold enough sludge to accommodate the higher influent flow rates in municipal/industrial facilities without having to enlarge the collector beyond economical size limits.
Moreover, large-capacity wastewater treatment systems, particular industrial and municipal systems, are often subject to large fluctuations in influent loads. For example, municipal sewage systems usually receive peak influent loads at morning, noon and early evening, with the influent flow generally being much lower during the rest of the day. The high flow rates which develop at peak loads can disrupt the sludge blanket and hamper the treatment process. Furthermore, the fluctuations in effluent render it difficult or impossible to maintain an optimal food-to-microbe ratio (referred to as the xe2x80x9cFTM ratioxe2x80x9d) when using a conventional single tank system, which is necessary in order to retain a microbe population which is large enough to fully metabolize the waste during peak periods: The is complete life cycle of the microbes spans only about twenty minutes, so that the system must contain enough food to support a large population over a several generations, through the many hours when there is little fresh food (influent) entering the system.
The microbes are capable of employing the sludge layer as food. In conventional single tank systems, however, it would be difficult or impossible to maintain sludge layer a large enough to support the microbe population while still being able to decant the clear liquid at a satisfactory rate, especially if influent flows were disturbing the sludge blanket during peak periods and causing the material to remain suspended in the water column.
Accordingly, there exists a need for a method and apparatus for single tank treatment of municipal, industrial, and other relatively high volume wastewater flows. Furthermore, there exists a need for such a method and apparatus which will permit the influent to enter the tank or other vessel at a relatively high volumetric rate without causing disruption of a sludge blanket which has settled to the bottom of the tank, and without requiring any complex baffle structure which could introduce eddies or turbulence into the flow. Still further, there exists a need for such a method and apparatus in which the amount of settled sludge material through which the influent must pass upon entering the tank is not constrained by the cross-sectional area of a comparatively small collector or chamber.
The present invention has solved the problems cited above and is a single tank wastewater treatment system in which a comparatively large amount of accumulated sludge is maintained so as to support large microbial populations between periods of peak influent loads, and in which there is a conically-tapered influent discharge structure which slows the velocity of the influent flow and discharges this against the bottom of the tank, so that the influent flows downwardly and outwardly under the sludge blanket without penetrating or dispersing the latter. The sludge blanket thus serves as a cap which prevents the fresh influent from xe2x80x9cshort-circuitingxe2x80x9d the system and passing directly to the decant assembly. The conical structure slows the flow gradually and smoothly by increasing the effective cross-sectional area of the influent supply conduit, and does not use any form of baffle which would introduce turbulence. The conical structure also has the advantage of presenting a minimal shadow zone in the upper portions of the tank, thereby causing less interference with effective mixing during the aeration phase, and less reduction of the effective volume of the tank, than would corresponding cylindrical or rectangular collectors or chambers.
The present invention also provides a method and apparatus for the treatment of manure-laden wastewater which is discharged from a confined animal operation, such as a feed lot, hog barn, or dairy farm, for example. The manure solids are separated from the wastewater flow before the wastewater enters the reactor tank. After treatment, the wastewater is returned to the facility for reuse, and the manure solids are disposed of in dry form as fertilizer.